Sudden Intermittent High Idle

[DPDISXR4Ti]

I know this is a common problem with Fords of this era, but figured I'd post it up here any way. High idle symptom appeared out of nowhere the other day - stayed high once warm and still occurred randomly after 50 mile drive. I'd say it's higher than when I've seen this condition before - maybe 2500 RPM (not exactly sure, as tach doesn't really work).

IAC was replaced semi-recently. Took it off to inspect and it looks very clean. I may replace it any way, but suspect that's not the issue.

Haven't pulled codes yet. Will do so, but I'm guessing none.

Details as they un-fold, but open to thoughts on whether there's anything Scorpio-specific that I haven't thought of. Just thinking out loud, I should take a look inside the PCM for leaking capacitors.

[brokencase]

Two cases...

First is a worn Throttle position sensor. It is a potentiometer. Since most of the time the throttle is around idle or just off of idle that is the portion of the resistor wiper that wears the most. If you put an old "needle style" ohm meter across the sensor and move the throttle you will see the needle "jump around" when the potentiometer is moved through the worn area.
This erronous input confuses the ECCIV and it forces the idle to a high setting.

Second, is a "voltage glitch" that caused the ECCIV to reset and go into "limp home mode".

I experienced the "voltage glitch" case when my A/C clurtch diode failed. This diode prevents the "back emf" of the A/C clutch engagement from upsetting the rest of the electrics of the vehicle. There is a TSB regarding the clutch diode on the Scorpio.
The main issue is that the connections to the diode get corroded. I soldered in a new diode.

[Ed Lijewski]

Congratulations! Scorpio Ownership initiation rites have begun.

There are posts about this phenomenon in Scorpio and in XR sections. New TPS didn't resolve issue in Scorpios, but seemed to in an XR.

YMMV

[brokencase]

I think it is the situation of the EECIV going into "limp home mode" for both cases...i.e., bad TPS sensor input OR unintended reset of the EECIV.

For other sensors the EECIV can workaround and set a code, but the TPS is a critical one so it has to fudge the idle on the high side of things.

DPDISXR4Ti - pull codes from the ECCIV if you have the reader. It may have logged the issue (then again it may not!).

FWIW - I carry a spare TPS in the glove box because I have been bitten by this back when I had my first Scorpio in the 90's.
This along with a spare TFI Ignition module.

[Ed Lijewski]

Limp Home mode doesn't allow engine to rev higher than modest rpms. Limp Home mode also allows operator to feather rpms from base idle up to the L.H. limit. In Sudden High Idle operator can raise rpms above the high idling point if needed (but in most cases operator shuts off engine and restarts it and S.H.I. vanishes.

YMMV

[DPDISXR4Ti]

No chance to work on this today since I was working on the aft end of the boat today. One bit of info I'll add is that I'm pretty sure Darren had already replaced the TPS, opting to go with the more readily available Ranger version that uses the typical American Ford quick-connector.

[Ed Lijewski]

Sheesh.

Ford provided new Scorpio.TPSs with gold plated terminals. Not so re Ranger stuff with regular naked metal of some sort. Scorpios merited top-notch service/repair consideration.

I installed a Ford one re dealing with Sudden High Idle.

YMMV

[DPDISXR4Ti]

One bit of info I'll add is that I'm pretty sure Darren had already replaced the TPS, opting to go with the more readily available Ranger version that uses the typical American Ford quick-connector.

My memory was wrong. Darren had indeed replaced the harness end, but used a clip from a donor harness, not new. The TPS itself was replaced with new Motorcraft.

Odds are that the TPS and IAC are not the problem, but I can't rule them out fully. Need to pull codes.

[Bob Weir]

A few thoughts for consideration"
1] Single ground wire at battery box is for the EEC
2] FI Harness Wiring
http://forum.merkurclub.net/forum/viewt ... fi+harness
3] Short to EEC

* Ken Kizer's Breakout Box - I think the club should buy it [if still available] and loan it to members. He made it and it's a big time saver!

[Ed Lijewski]

What will it take for brokencase to ask this expert for his take on cause and solution to SHIS?

viewtopic.php?f=19&t=33259

YMMV

[Bob Weir]

viewtopic.php?f=19&t=33259&start=30

[DPDISXR4Ti]

KOEO Error codes pulled today...

21 Engine Coolant Temperature (ECT) sensor out of range
41 System was lean for 15 seconds or more (no HO2S switching)
52 Power Steering Pressure Switch/circuit open

21 and 41 could be related. I guess I'll replace the ECT.

52 I'm not worried about

[Bob Weir]

Brad
Did u open the wiring harness?

Do you have a source for the allowable ± variance of ohm and voltage of the engine management sensors?

[DPDISXR4Ti]

Did u open the wiring harness?

Funny enough, I was just talking to Darren about that tonight. I have not yet opened up the harness, but indeed Darren did, and made a repair on the ECT circuit. He had already replaced the sensor itself, so doubtful that's the problem. I'll be investigating further.

Has anyone ever considered replacing the whole harness with one from a 90's Ranger perhaps?

[Ed Lijewski]

Two digit codes (1984-88) say 41 indicates system always lean which might suggest 02 sensor.

YMMV

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